The present invention relates to heating systems for drying wet coatings such as printing inks, paint, sealants, etc. applied to a substrate. In particular, the invention relates to a drying system in which a blower having an inlet directs a current of heated gas such as air towards a wet coating on a substrate to dry the coating and wherein the heated air is circulated back to the inlet of the blower once the air impinges the coating on the substrate. The present invention also relates to a drying system in which the substrate is supported about a thermally conductive roll having a plurality of energy emitters disposed within the conductive roll along a length of the conductive roll. The plurality of energy emitters are controlled to selectively emit energy along the length of the conductive roll. The dryer system preferably includes means for sensing temperatures of the roll along the length of the conductive roll, wherein the energy emitted by the energy emitters along the length of the roll varies based upon the sensed temperatures along the length of the roll.
Coatings, such as printing inks, are commonly applied to substrates such as paper, foil or polymers. Because the coatings often are applied in a liquid form to the substrate, the coatings must be dried while on the substrate. Drying the liquid coatings is typically performed by either liquid vaporization or radiation-induced polymerization depending upon the characteristics of the coating applied to the substrate.
Water or solvent based coatings are typically dried using liquid vaporization. Drying the wet water-based or solvent-based coatings on the substrate requires converting the base of the coating, either a water or a solvent, into a vapor and removing the vapor latent air from the area adjacent the substrate. For the base within the coatings to be converted to a vapor state, the coatings must absorb energy. The rate at which the state change occurs and hence the speed at which the coating is dried upon the substrate depends on the pressure and rate at which energy can be absorbed by the coating. Because it is generally impractical to increase drying speeds by decreasing pressure, increasing the drying speed requires increasing the rate at which energy is absorbed by the coating.
Liquid vaporization dryers typically use convection, radiation, conduction or a combination of the three to apply energy to the coating and the substrate to dry the coating on the substrate. With convection heating, a gas such as relatively dry air, is heated to a desired temperature and blown onto the coating and the substrate. The amount of heat transferred to the substrate and coating is dependent upon both the velocity and the angle of the air being blown onto the substrate and the temperature difference between the air and the substrate. At a higher velocity and a more perpendicular angle of attack, the air blown onto the substrate will transfer a greater amount of heat to the substrate. Moreover, the amount of heat transferred to the substrate will also increase as the temperature difference between the air and the substrate increases. However, once the substrate obtains a temperature equal to that of the temperature of the air, heat transfer terminates. In other words, the substrate will not get hotter than the air. Thus, the temperature of the air being heated can be limited to a level that is safe for the substrate.
Although controllable, convection heating is thermally inefficient. Because air, as well as nitrogen, have very low heat capacities, high volumes of air are required to transfer heat. Moreover, because the heated air blown onto the coating and substrate is typically allowed to escape once the heated air impinges upon the coating and the substrate, conventional drying systems employing convection heating typically use extremely large amounts of energy to continuously heat a large volume of outside ambient air to an elevated temperature in order to provide the high volumes of flow required for heat transfer. Because convection heating requires extremely large amounts of energy, drying costs are high.
Radiation heating occurs when two objects at different temperatures in sight are in view of one another. In contrast to convection heating, radiation heating transfers heat by electromagnetic waves. Radiation heating is typically performed by directing infrared rays at the coating and substrate. The infrared radiation is typically produced by enclosing electrical resistors within a tube of transparent quartz or translucent silica and bringing the electrical resistors to a red heat to emit a radiation of wavelengths from 10,000 to 30,000 angstrom units. The tubes typically extend along an entire width of the substrate.
The last method of applying energy to a coating and a substrate is through the use of conduction. Conductive heating of the coating and substrate is typically achieved by advancing a continuous substrate web about a thermally conductive roll or drum. Hot oil or steam is injected into the drum to heat the drum. As a result, the heated drum conducts heat to the substrate in contact with the drum. Because the drum must be configured so as to contain the hot oil or high pressure steam, the drum or roll is extremely complex and expensive to manufacture. In addition, because of the large mass of the drum required to accommodate the oil or high pressure steam, the dryer system employing the drum often requires a complex drive mechanism for rotating the heavy drums or rolls. This complex drive mechanism also increases the cost of the drying system. Moreover, because the oil or hot steam uniformly heats the thermally conductive drum across its entire length, the thermally conductive drum uniformly conducts energy or heat along the entire width of the substrate in contact with the drum regardless of varying drying requirements along the width of the substrate due to varying substrate and coating characteristics along the width of the substrate. As a result, portions of the substrate which do not contain wet coatings or which contain coatings that have already been dried unnecessarily receive excessive heat energy which is wasted. Conversely, other portions of the substrate containing large amounts of wet coatings may receive an insufficient amount of heat energy, resulting in extremely long drying times or offsetting of the wet coatings onto surfaces which come in contact with the wet coatings.